The lewis structure of trinitramide has four major resonance structures, whereby the lone pairs of the central nitrogen can resonate with the nitro groups.
This seems to indicate that each of the nitrogen atoms would be sp2 hybridized, and the entire molecule would be planar. In fact, the Wikipedia ball-and-stick image for trinitramide seems to indicate it so.
However, based on quantum mechanical calculations, trinitramide is in fact non-planar. Why is this so? What drives its non-planarity?